The present invention relates to apparatus for making remote distance measurements that has particular applicability to motor vehicles. The invention uses at least one electroacoustic transducer to transmit an ultrasonic signal and to receive the ultrasonic signal reflected by an object. An electric generator is used to activate the transducer. A receiving stage is provided for the echo signals picked up by the transducer. A control unit uses the generator to activate the transducer for a predetermined transmission time. The transducer then decays for a time following its activation to provide a reception window for receiving the reflected echo signals.
Devices for remote distance measurement of the foregoing type are known from West German Patent 3,513,270. This reference describes an apparatus for measuring distance in which a control unit can vary the transmission time and/or the transmission power of the transducer. This reference further provides for making the time duration of the reception window smaller, as measured from the start of the transmitting time, for a cycle having a shorter transmitting time and/or a transducer having smaller transmitting power than for a cycle having a longer transmitting time and/or a transducer having a larger transmitting power. The voltage amplitude at the input of the transducer is varied which results in an equivalent change of the delay time when the attenuation of the transducer is kept constant. Reducing the voltage amplitude, however, decreases the useful signal at shorter object distances.
West German Patent No. 430,461, discloses a range o measuring method that uses reflected pulses from a mechanical transmitter. The transmitter has a resonance frequency that is slightly different than the resonance frequency of the receiver. The transmitter is driven at the resonance frequency of the receiver. The transmitter decays quickly once its energy supply is stopped. The frequency of the transducer then shifts away from the resonance frequency of the receiver. A switching pulse is thus generated having a well defined frequency and duration.